Optical or other electromagnetic waves which propagate within a closed system may be used to measure the rotation rate of an optical gyroscope without references to outside information. All such gyroscopes are based on the fundamental physics of electromagnetic wave propagation within a rotating system. When a system composed of an optical or electromagnetic source, a propagation path, a medium in which the light waves propagate, and detectors for the light waves are all rigidly mounted with respect to each other but which rotate together as a system, then because of the fundamental physics of such a system, there are measurable effects on the detected waves that permit a user to sense the rate of rotation and the direction of rotation.
There are several manifestations of the fundamental physics of these systems. The best known is the Sagnac effect, which is the basis for several types of existing optical gyroscopes. To induce the Sagnac effect, two optical waves propagate in opposite directions along a closed, generally circular or triangular, coplanar path. It is important that the two counter-propagating waves follow the identical path except in opposite directions. The relative phase between the two waves may be sensed by means of detectors of the interference pattern between the two waves. When there is constant rotation of the system about an axis perpendicular to the plane of the closed optical path, then there is a shift in the relative phase between the two waves, and this shift is proportional to the rate of rotation both in magnitude and sense of direction.
The best known of the practical gyroscopes based on the Sagnac effect is the ring laser gyroscope (RLG). In such a gyroscope, the optical path is filled with a lasing medium providing the light source, and the phase shift of the light manifests itself in a frequency difference between the two counter-propagating waves. For a review of ring laser gyroscopes see, for example, J. E. Killpatrick, "The Laser Gyroscope", IEEE Spectrum, October 1967, vol. 4, pp. 44-55; F. Aronowitz, "The Laser Gyro", in Laser Applications, Vol. 1, M. Ross, Editor, Academic Press, N.Y. 1971, pp. 113-200; and for a more theoretical treatment W. Chow, et al., "The Ring Laser Gyroscope", Reviews of Modern Physics, Vol 57, January 1985, pp. 61-104. A review of the history, of some relevance to the concept described here, is given by C. V. Heer, "History of the Laser Gyro", in the Proceedings of the SPIE, Vol. 487, Physics of Optical Ring Gyros, (Conference, Snowbird, Utah, Jan. 7-10, 1984.), Pub. by SPIE, Bellingham, Wash.; pp. 2-12.
The Sagnac effect has also been exploited in the fiber optic gyroscope (FOG), which differs from the RLG in both the means for providing an optical path and the use of light from sources external to the circular path. In the FOG, the Sagnac phase shift is measured more directly rather than measuring a frequency difference. The FOG is nearing engineering and commercial success. For a review see, for example, R. A. Bergh, et al., "An Overview of Fiber-Optic Gyroscopes", IEEE J. of Lightwave Technology, Vol. LT-2, No. 2, pp. 91-107. An extensive reprint collection of the significant publications and a bibliography has been published. R. B. Smith, Selected Papers on Fiber-Optic Gyroscopes, SPIE Milestone Series Vol. MS 8, published by SPIE Optical Engineering Press, Bellingham, Wash. 1989. A recent topical book is H. Lefevre, Fiber Optic Gyroscopes, Artech House, Mass. 1993.
It is notable that all successful optical gyroscopes to date are based on the Sagnac effect where the light propagates around a closed circular path. Because of this, there is virtually no precedent for rotation sensors based on other possible manifestations of the effects of rotation on electromagnetic wave propagation in rotating systems. Once the Sagnac effect is understood and described, there is no need to return to the fundamental physical laws that give rise to the effect. Consequently in nearly all the publications which describe RLGs and FOGs, there is no description of rotation sensors in which the light propagates in directions other than around closed paths which are generally perpendicular to the axis of rotation.
The concept disclosed here does not depend on the Sagnac effect in that the light or electromagnetic radiation does not propagate along a closed planar path but rather in a direction parallel to the axis of rotation of the gyroscope. Therefore, there appears to be little precedent in the traditional prior art of ring laser or fiber-optic gyroscopes, despite their maturity. Instead, relevant precedent publications appear in earlier descriptions of the physics related to rotation on electromagnetic wave propagation.
The publication by C. V. Heer cited above, "History of the laser gyro", reviews the earliest proposals for RLGs. Two other publications which describe the fundamental physics are: E. J. Post, "Sagnac Effect", Reviews of Modern Physics, Vol. 39, No. 2, April 1967: pp. 475-493., and E. J. Post, "Interferometric Path-Length Changes Due to Motion", J. of the Optical Society of America, Vol. 62, No. 2, February 1972; pp. 234-239. The fundamental treatment in these two articles is general enough to be the basis for describing nearly any effect due to rotation, although the emphasis is on the Sagnac effect. E. J. Post emphasizes that the effects are examples of general relativity, as well as the dependence on the optical properties of the transparent material within which the light propagates.
C. V. Heer, "Resonant Frequencies of an Electromagnetic Cavity in an Accelerated System of Reference", Physical Review, Vol. 134, No. 4A, 18 May 1964, pp. A799-A804, predicted the physical effect on microwaves in a closed cylindrical resonant cavity with the axis of rotation being parallel to the axis of the cylinder. Heer implied that it may be possible to use this physical effect (based on original work by Fermi) to build a sensor to detect angular velocity, but no such device is known to have been proposed in structure or actually built.
This previously described Fermi effect was not experimentally verified until R. V. Jones, "Rotary `Aether Drag`, " Proc. Royal Society, London, Vol. A349, 29 Jun. 1976, pp. 423-439. Jones built a device with a rotating cylinder through which the electromagnetic radiation propagated, but it was not a gyroscope. Further, Jones did not describe the effects and importance of light passing through a medium.
An early patent does describe the use of microwave resonant cavities for rotation sensing, J. B. Speller, "Relativistic Inertial Reference Device", U.S. Pat. No. 3,395,270, issued Jul. 30, 1968, filed Jun. 28, 1962. However, in all of the structures disclosed by Speller, the resonant cavity and the wave path are all toroidal shaped with a circular path, corresponding exactly to the Sagnac effect and not anticipating any devices in which the propagation path is directed parallel to the rotation axis of the device.
Some further background to the present invention is provided by additional work and publications, which discuss the effects of rotation on light propagating along the axis of a cylindrical system wherein the material is rotating with respect to either the light source, the detectors or both. However, none considered any case where the light source, material medium, and the detectors are all fixed with respect to each other, and rotating together as a system. In some cases, reference is made to the Coriolis contribution to the effects of the rotating material. This is a further effect truly due to rotation in inertial space. However, it is an effect acting on the electrons or the material properties of the propagation medium, and not considered acting on the light. It is not proposed by any of the authors for rotation sensing or gyroscopes, including these authors:
R. V. Jones, "Rotary `Aether Drag`, " Proc. Royal Society, London, Vol. A349, 29 Jun. 1976, pp. 423-439. PA0 M. A. Player, "On the Dragging of the Plane of Polarization of Light Propagating in a Rotating Medium" Proc. Royal Society, London, Vol. 349, 1976, pp. 441-445. PA0 N. B. Baranova and B. Ya. Zeldovich, "Coriolis contribution to the Rotary Ether Drag", Proc. Royal Society, London, Vol. A368, 1979, pp. 591-592. PA0 J. P. Woerdman, G. Nienhuis, and I. Kuscer, "Is it possible to Rotate an Atom?", Optics Communications, Vol. 93, No. 1-2, 15 Sep. 1992, pp. 135-144. PA0 G. Nienhuis, J. P. Woerdman, and I. Kuscer, "Magnetic and Mechanical Faraday Effects", Physical Review A, Vol. A46, No. 11, 1 Dec. 1992, pp. 7079-7092. PA0 N. B. Baranova, B. Ya. Zel'dovich and J. P. Woerdman, "Can an Atom be Set in Rotation?", JETP, Vol. 77, No. 3, Sep. 1993, pp. 379-381. (English translation of Zh. Eksp. Teor. Fiz., Vol. 104, September 1993, pp. 2969-2974. Journal title translates as Journal of Experimental and Theoretical Physics, hence J.E.T.P.)